Conventionally, in the case where a steel part is heat-treated, a high frequency induction heating apparatus has been used. In tempering treatment after quenching treatment as well, the high frequency induction heating apparatus has often been used. As a publicly known technique of the high frequency induction heating apparatus, for example, Japanese Unexamined Patent Application Publication No. 2001-303134 is cited.
Conventionally, in the case where a steel part is quenched and tempered, a high frequency heat treatment apparatus 50, for example, shown in FIG. 3 is usually used. A process for performing heat treatment by using this high frequency heat treatment apparatus is as described below. First, in a quenching treatment section 2, an object 1 to be treated is heated to a required quenching temperature with a predetermined high frequency power for a predetermined period of time by using a quenching high frequency induction heating coil 3 connected to a quenching high frequency power source, not shown. Then, by spraying a quenching coolant 5 on the object 1 to be treated from a cooling means 4, the object 1 to be treated is rapidly cooled to a 90% martensite transformation finishing temperature (about 215° C., depending on the material properties of the object 1 to be treated). Hereunder, this rapid cooling process is referred to as a primary cooling process.
Thereafter, in a secondary cooling section 6 located at a position different from the position at which the primary cooling process is performed, the object 1 to be treated that has been subjected to 90% martensite transformation is sprayed with a quenching coolant 5 from a cooling means 7 so that the whole of the object 1 to be treated is cooled to the ordinary temperature for quench heating. Hereunder, this cooling process for performing soaking to the ordinary temperature is referred to as a secondary cooling process. The quenching coolants 5 sprayed in the primary and secondary cooling processes are an identical liquid (coolant) supplied from a common quenching coolant tank, not shown.
In an air blow section 8, by spraying compressed air 10 on the object 1 to be treated in a chamber 9, the quenching coolant 5 adhering to the surface of the object 1 to be treated that has cooled down to the ordinary temperature in the secondary cooling process is removed, and then, the object 1 to be treated is conveyed to a tempering treatment section 11 by using a conveying apparatus, not shown. Next, the object 1 to be treated is temper heated with predetermined high frequency power for a predetermined period of time by using a tempering high frequency induction heating coil 12 of the tempering treatment section 11, which is connected to a tempering high frequency power source, not shown. Then, the object 1 to be treated is air-cooled (let cool) for a required period of time on a cooling jig (a member for holing the object to be treated to let it cool) 13 arranged at a position separate from the position of the tempering high frequency induction heating coil 12, by which the quenching treatment and the tempering treatment are finished. Thus, for example, as shown in FIG. 4, a hardened layer S obtained by high frequency quenching treatment is formed in a hatched region of the outer peripheral surface of a columnar part α of an inner ring W of a hub unit, which is the object 1 to be treated. In FIG. 3, reference numerals 14, 15, 16 and 17 denote members for holding the object to be treated, which is used to hold the object 1 to be treated in a state of being placed at respective predetermined positions in the quenching treatment section (primary cooling section) 2, the secondary cooling section 6, the air blow section 8, and the tempering treatment section 11, respectively.